The invention relates to induction motor drives in general, and more particularly to an improved mode of deriving information relative to the motor slip frequency with a view of optimizing performance and control of a motor drive.
Since the adjunction of solid state motor control the induction motor has become the ideal motor for a drive because of its ruggedness and simplicity. While maximum motor efficiency and optimal drive control require knowledge of a number of operative parameters of the motor, in the induction motor the only readily available ones are the voltage applied to the stator and the stator and motor currents. Without more, though, it is not possible to ascertain what portion of the current is allocated to building of the flux for stator-rotor coupling and what portion actually delivers power on the shaft to meet the torque. In that respect the slip frequency of the motor, (e.g. the difference in angular velocity existing, in the presence of a torque, between the rotation in space of the electrical vector representing the magnetomotive force of the stator and the angular rotation of the driven shaft) constitutes an important performance characteristic of the motor. In particular, there exists an univocal relationship between the slip frequency and the stator current for a given flux and torque, regardless of the other electrical or mechanical quantities. Therefore, knowledge of the slip level and of the stator current is sufficient to completely characterize the behavior of the motor.
When the induction motor is used in conjunction with variable frequency power supplies, such as solid state inverters or cycloconverters, it is important to know the slip of the motor at any instant in order to be able to control the static and dynamic behavior of the drive. First, knowledge of the slip gives a measure of how to compensate for the natural droop in speed with load which is characteristic of an induction motor. As a result close speed regulation is made possible. Secondly, the knowledge of the slip permits an optimal control of the drive, mainly the magnitude (and sign) of the generated torque and the degree of excitation of the machine, e.g. the flux level, since as mentioned above the slip reveals univocally these two parameters.
In the prior art the slip has been measured by mechanical or electromechanical rotary devices coupled to the motor shaft, and such measurement has been used to improve the dynamic performance of a drive. (See for instance F. Blaschke and G. Huetter, "Inverter for Control of Induction Machines in Motoring and Regenerative Drives", in ETZ-A (Germany) Vol. 89 (1968), part 5, pp. 108-112. See also P. G. Cushman and A. A. Clark, "Co-ordinated Voltage Control for Induction Servomotors," U.S. Pat. No. 3,700,986, October, 1972.)
In particular, it is known to feedback a signal indicative of the slip for controlling a "current fed" inverter drive, also called adjustable current input drive (ACI). See for instance K. P. Phillips, "Current Source Converter for AC Motor Drives," IEEE Conference Record of IGA/1971 Meeting, pp. 385-392. See also M. B. Brennen, "A Comparative Analysis of Two Commutation Circuits for ACI Inverters Feeding Induction Motors," IEEE Conference Record of PESC/73 Meeting, pp. 201-212. In such instances the inverter circuit routes the current generated by the front end converter, or chopper, through the motor windings, at a controlled sequence and frequency. This current is regulated by converter control in response to the level of the slip in order to achieve a programmed interdependence between stator current and slip level. The regulation program can be chosen to achieve under any load situation and at any output frequency, a desired level of flux, usually the nominal flux defined by the machine nameplate voltage and frequency.
The usual means by which a slip signal is obtained is through the use of electromechanical rotary transducers coupled to the motor shaft. The simplest arrangement, commercially used in ACI drives, is a tachometer coupled to the shaft for deriving induction of the shaft speed. The tachometer signal subtracted from the analog frequency command to the inverter yields information on the slip. This method requires the use of a rather precise tachometer, since even small relative errors between the two terms of the subtraction greatly affect the difference. In particular, the tachometer introduces a ripple which can be too important when fast loop response is desired. Pulse tachometers have been used implementing the subtraction digitally and thus eliminating problems of inaccuracy. Differential electromechanical transducers (See: T. M. Corry "Triggering Circuit for a Controlled Rectifier," U.S. Pat. No. 3,413,493) or even geared differential mechanisms (See: P. D. Agarwal "The General Motors High Performance Induction Rotor Drive System" IEEE Transactions on Power Apparatus and Systems, Vol. PAS-88 No. 2 pp. 86-93 (1969) have been considered as a substitute solution. Such systems, however, have two major drawbacks. First, they impair the mechanical interface between the motor and the load by interfering with the requirements of tight coupling or close spacing in the mechanical layout, or by requiring the adjunction of undesirable gear trains or belts. Secondly, they generally require the use of additional organs such as brushes, slip rings, light sources which introduce in the drive system a weak link from the viewpoint of overall reliability. In short, such systems lack the simplicity and ruggedness which are the most attractive qualities of an AC motor drive. A method by which a signal proportional to the motor slip frequency would be derived without the use of any rotary transducers of any sort is therefore highly desirable.
The main object of the present invention is to provide an induction motor drive in which an indication of the slip is derived at all times only by electrical circuitry, thus involving no mechanical or electromechanical device, nor the adjunction of any contrivance between the stator and rotor or on the driven shaft of the motor.
Another object of the present invention is to provide an improved mode for deriving an indication of the slip of an induction motor.
A further object of the present invention is to derive a slip signal of an induction motor drive through analog sensing of the currents applied to the stator and the currents flowing in the stator of the induction motor and through analog circuitry combining the sensed signal.
Still another object of the present invention is to provide an improved static controlled motor drive of the induction motor type in which the slip frequency of the motor is used as a control parameter.